The blast, which struck buses of people who were leaving their towns as part of a rebel-regime swap, also injured 55 others in Rashidin, a suburb of Aleppo in northwestern Syria, according to Syria Civil Defense, also known the White Helmets.

The convoy of buses, which were parked at the time, was carrying thousands of people from two regime-held but rebel-besieged villages in northwestern Syria, state-run media reported.

Video shown on Syrian state-run television showed heavily damaged and burned buses parked on the side of a road. People walked outside the buses, surveying the damage as well as bodies lying on the roadway and a grass median.

The evacuees, from the mainly pro-regime Shia villages of Al-Fu'ah and Kafriya, were bound for regime-held parts of Aleppo.

The state-run Syrian Arab News Agency reported the convoy continued, and the first buses arrived late Saturday in Aleppo. The buses headed to the Jebrin area for a temporary housing center equipped with food and medical supplies, SANA said.

The rebel group Ahrar al-Sham tweeted that some of its members died in the blast. They were at the site to ensure the convoy's passage, Ahrar al-Sham said. The group said it was investigating to find out who was responsible.

As part of the deal, government forces are allowing thousands of rebels and civilians to leave two towns in southwest Syria: Madaya and Zabadani, according to the London-based Syrian Observatory for Human Rights.

State-run media: Rigged car was disguised

The explosion happened as both sets of evacuees were stopped in separate locations outside Aleppo. Each were heading to areas controlled by forces friendly to them.

The explosives-rigged car had been packed with children's food supplies, perhaps to disguise it, a correspondent with SANA reported.

The convoy that departed Al-Fu'ah and Kafriya is carrying 5,000 people, the Syrian Observatory for Human Rights said.

Thousands were leaving Madaya and Zabadani, including more than 2,000 rebel fighters, their families and other civilians, the monitoring group said.

The deal was brokered by Iran and Qatar, Agence France-Presse reported.